A routine sweep to make a range safe for future training exercises at California's Camp Pendleton led to the deaths of four Marines who were trying to clear unexploded ordnance, a Marine official said.

There was no live firing on the range at the time of the periodic sweep of explosive materials, said a Marine official who spoke on condition of anonymity because he was not authorized to speak publicly.

Base officials released no details on the 11 a.m. accident. Authorities were investigating the cause.

The identities of the dead were withheld pending notification of relatives.

"We offer our heartfelt prayers and condolences to the families of the Marines lost today in this tragic accident," said Brig. Gen. John W. Bullard, commanding general of Marine Corps Installations West at Marine Corps Base Camp Pendleton. "Our first priority is to provide the families with the support they need during this difficult time."

The deaths come about eight months after a mortar explosion killed seven Marines during a live-fire training exercise in Nevada.

A military investigation determined human error was to blame for that accident. According to the probe's findings, a Marine operating a 60 mm mortar tube and ammunition did not follow correct procedures, resulting in the detonation of a high explosive round at the mortar position.

The investigation also determined the mortar team involved in the accident had not conducted "appropriate preparatory training."

(Copyright 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.)